This invention relates generally to load supporting constructions such as roof truss structures and more particularly, provides an improved wood roof truss construction formed of a pair of redundant superpositioned load resisting systems in additive combination, namely, a tied-arch system superpositioned upon a determinate wood bowstring truss system, said systems consisting of top and bottom chords, interconnecting web formation, the top chord being common, said truss including a horizontally oriented tension rod connected only to the bearing ends of the wood truss and not to the bottom chord.
Therefore are many and diverse competing load resisting (supporting) structural systems, both in bridge design and building construction. Systems like the tied cresent-arch system comprise a pair of top arched chords secured at their ends and having differing radii of curvature. A web structure bridges the space between the top arched chord pair along the length thereof. A horizontally oriented tie-rod bridges the ends of said arched chords. Vertically oriented rods are secured between the lower one of the arched chords and the tie-rod. This structure employs an inordinate number of curved members, has complex geometry and is too costly.
A three-hinged trussed arch is another well-known load-resisting support structure. This construction involves an upper arch formed of a pair of bowstring truss structures joined end to end to define the upper arch. The free ends of such arch are bridged by a horizontally oriented tie-rod or bottom chord, same being coupled to and supported from the upper arch by vertically oriented hanger rods fixedly secured thereto. This system also is complex geometrically and very expensive to construct and install.
The conventional tied-arch load supporting system is employed for bridging spans in the range of 50 to 80 feet for relatively small building constructions. The conventional tied arch system comprises, generally, a continuous wood arch member functioning as the top chord and a bottom chord comprising a tie rod secured to the top chord at opposite ends. Vertically oriented, spaced hanger rods are secured to the tie rod along the length of the tied-arch. The quantity of wood required for this system is substantial because a large quantity of wood is required to form a stable arch capable of resisting severe unbalanced loading. For example, the volume of wood required to form the top chord of a tied-arch system is about three times as much as that required to provide the top chord of the conventional bowstring wood truss. These members must be formed of glue-laminated wood members, costing in the range of three to five times the cost of common cut-lumber. One advantage of the tied-arch system is the absence of distracting web structures as a mar to its appearance as compared to the appearance of the conventional bowstring truss conventionally dominating industrial applications.
The conventional wood bowstring truss construction employs a wood top arch similar to that of the tied-arch system and a wood bottom chord secured to the ends of the top arch. A web array is secured between the arch and the bottom chord, the web array consisting of vertically oriented planks located at spaced points along the length of both arch and bottom chords. Angularly oriented wooden web members also are secured to said top arch and bottom chord, the junctures along the arch being generally equally spaced, while the junctures along the bottom chord being adjacent to the vertical plank juncture with the bottom chord as a gusset arrangement. The end seating requires massive joints.
Serious functional disadvantages have been encountered in the use of the bowstring type wood truss system per se. Among these are functional failure under stress, e.g. leading to collapse under the weight of heavy snow. Many of the adverse experiences of bowstring wood truss can be traced to defects in the bottom chord, bad knots in the lumber or know clusters or severe slant of grain in the lumber. Rotting at the ends have been encountered leading to sagging of the truss with resulting bending and fracture of the bottom chord. The truss systems of the bowstring type as mentioned, in addition, require unusually long, high quality timbers not readily available in large quantities. Further, massive heavily bolted wood-plank splices are required for timber tension members, a dominating cost factor.
In view of wood truss functional failures that have been encountered, more recent governing design codes have reduced the allowable tension stress in timbers considerably. Design snow loads have been increased by about fifty percent, and in some jurisdictions, even greater loads are required by local building codes. The net result has been an elimination of the cost gap between the wood bowstring truss and the flat steel bar joist system or the rigid steel frame system to which the art has turned as a replacement for wood truss systems so described.
Thus a considerable need has arisen to provide a less expensive, yet structurally improved, load support system capable of construction in situ in the field from stock lumber etc. delivered to the site of erection. Such need is sought to be fulfilled by the hereinafter disclosed invention.
Accordingly, the invention herein provides a wood roof truss construction capable of functioning at least to accepted standards yet being capable of erection with substantial material and labor cost savings.